The present invention relates to an electrophotographic color image forming apparatus in which a plurality of image exposure means (an optical exposure system) are provided in an image forming member (a photographic drum), a plurality of developing means are arranged along the circumferential surface of the image forming member, and toner images are superimposed on the surface so that a color image is formed while the image forming member is rotated one rotation.
As a method of forming a multi-color image, there have been known several techniques using the following apparatus:
An apparatus (A) in which photoreceptor drums, charging devices and developing devices each corresponding in number to colors necessary for forming the multi-color image are provided, and a mono-color toner image formed on each photoreceptor drum is superimposed one after another on a transfer drum so as to form a color image.
An apparatus (B) in which a single photoreceptor drum is rotated plural times so that the charging, image exposure and developing processes are repeated for each color so as to form a color image.
An apparatus (C) in which the charging, image exposure and developing processes are conducted for each color sequentially so as to form a color image while a single photoreceptor drum is rotated one rotation.
However, the apparatus (A) has a drawback that the dimensions of the apparatus become too large because the plurality of photoreceptor drums and the transfer drum are required. The dimensions of the apparatus (B) can be made relatively smaller because the required number of each of the charging means, image exposure means and photoreceptor is only a single, however, the apparatus (B) has a limitation that the size of a formed image is limited not to be larger than the area of the circumferential surface of the photoreceptor drum.
The apparatus (C) makes it possible to form images at a high speed.
However, the apparatus (C) has a restriction in a structural arrangement that the plurality of charging devices, image exposure means and developing devices must be arranged within a circumferential area of the photoreceptor drum. Further, the apparatus (C) has a problem that an image exposure means to conduct an image exposure is apt to be soiled with toners leaking from a developing device neighboring the image exposure means and may resultingly degrade an image quality. In order to avoid this problem, it is required that the distance between the image exposure means and the developing device is to be made larger. With this requirement, the apparatus (C) has a contradiction that the diameter of the photoreceptor drum inevitably become large, resulting in that the apparatus also inevitably become large.
For the purpose of eliminating the above contradiction in the apparatus (C), an apparatus, in which the base member of a photoreceptor drum is made of a transparent material and a plurality of image exposure means are incorporated in the photoreceptor drum so that an image is exposed from the image exposure means through the base member onto a photosensitive layer on the external surface of the base member, has been suggested (for example, Japanese Patent Publication Open to Public Inspection No. 307307/1993).
However, the apparatus according to the above suggestion has drawbacks that, as shown in FIG. 6, in the event that an image exposure means 12 provided on a supporting member 20 is arranged inside an image forming member (a photoreceptor drum) 10 and the image forming member 10 is exposed from its inside, exposure lights are reflected from a toner layer T, come again into a photosensitive layer 10c of the image forming member 10, and are scattered in the photosensitive layer 10c so that a latent image is agitated with the scattered lights and a clear image having a good color balance can not be formed.
The first object of the present invention is to eliminate above drawbacks and to provide a color image forming apparatus capable of obtaining a clear image having a good color balance.
Further, in an apparatus in which an image exposure means is provided in an image forming member, since a photosensitive layer is exposed again with reflected lights which have been emitted to an inside of the apparatus after passing through the photosensitive layer and have been reflected on the inside of the apparatus, a latent image formed on the photosensitive layer is inevitably affected by the re-exposure to some extent.
Still further, in the event that an image forming member on which a toner image has been already formed is further image exposed, some toners on a part of the toner image at a position on which the image forming member is electrically discharged by the exposure may become a floating condition so that there is a risk that the toners may be scattered.
Still further, an additional exposure process is conducted for an image forming member in order to eliminate remaining charge with an exposure light before a charging process to charge a photosensitive layer, during a transferring process to transfer a toner image, or before a cleaning process to remove residual toner remaining after the transferring process, whereby a uniform electric potential can be applied, a transfer rate can be increased or the residual toner can be removed easily. However, if useless reflected lights are generated by the additional exposure processes, the reflected light further cause a latent image having been formed on the image forming member to degrade or toners to scatter in the apparatus, similar to the problem in the image exposure process mentioned before.
The second object is to effectively prevent exposure lights from repeatedly exposing a photosensitive layer and from causing toner to scatter on each exposure process conducted during image exposure, before charging, simultaneously with the transfer process, or before the cleaning process by solving the above problems and by making an improvement, and, as a result, to provide a color image forming apparatus capable of forming a high grade image.